Well I suppose it depends on the situation...
In most cases I would say the only advantage to using a high res layout would be if you want a large list, or multiple artwork types displayed. So I think you're probably OK at 640x480, especially if you run all your emulators at 640x480 too.
Something else to note is in theory the higher the res, the worse MAMEWAH will perform, tho this will also depend on how complex a layout you're using...
Personally I use 368x240 on my arcade monitor, and (apart from testing) 640x480 on a PC monitor.